George R.R. Martin's A Game of Thrones is the first volume of what is the absolute best current running fantasy series. It's three books into what will be a six book run.

For some awesome time travel, Connie Willis' amazing Doomsday Book and its comedic sequel To Say Nothing of the Dog are excellent both for entertainment value, as well as for the amazing research she put into both the science and the history.

Some amazing sci-fi (although the alien culture is a little too humanlike) -- Vernor Vinge's A Fire Upon the Deep and its sequel/prequel A Deepness in the Sky are pretty good if you can get into them.

I don't know if you want to get into long series, but the first four or five books of Robert Jordan's Wheel of Time, starting with The Eye of the World are excellent, and the rest are pretty good, but the series won't be done for another 5-8 years. Terry Goodkind's Sword of Truth, starting with Wizard's First Rule, is exceptionally readable, and each book stands on its own. However, his morals are a bit heavyhanded.

Other good sci-fi Ender's Game by Orson Scott Card, Armor by John Steakley, Hyperion and its sequel, which are really two parts of the same book, are outstanding (by Dan Simmons).

Of course, no fantasy reader would be complete without Lord of the Rings by Tolkien.

For excellent contemporary fantasy/sci-fi mix, check out The Iron Dragon's Daughterby Michael Swanwick and Heroes Die and its sequel Blade of Tyshalle by Matthew Woodring Stover. These guys will blow you away.

There's tons of other good stuff, too, but it's such a bitch sorting the good from the crap.

It would be helpful if you gave us some examples of books you like so that we could recommend similar books...

Why do you need any from series fiction (like Star Wars or Star Trek) at all?

The Lord of the Rings by J. R. R. Tolkien. Practically gave birth to the modern fantasy genre. LotR is a single novel, not a series, that is usually (but not always) offered in three volumes.

The Dark Tower: The Gunslinger by Stephen King. Granted, this book introduces his Dark Tower series, but it stands up well on its own. The Dark Tower is a bizarre combination of horror, science fiction, and fantasy with a bit of the western tossed in.

Stranger in a Strange Land by Robert Heinlein. Perhaps his most controversial novel, the story of Valentine Michael Smith, the human raised by Martians who becomes a Messiah-type figure on his home world, is still a classic.

A Canticle for Liebowitz by Walter M. Miller, Jr. A literary classic in its own right. The Catholic Church in the dark ages of post-Holocaust America.

Darwin's Radio by Greg Bear. A highly controversial look at biotechnology and the mechanisms of evolution. Solid, hard SF.

Dune by Frank Herbert. Space opera at its finest. The struggle for the planet whose only export, spice, is a necessary part of civilization. Again, the beginning to a longer series, but a book that stands alone by itself.

That's six books. But they'd provide enough material for almost any SF/Fantasy unit.

Read Ender's Game, by Orson Scott Card
For science-fiction, you can't say enough about this novel.Heroes Die, by Matthew Woodring Stover
This novel will kick you in the nuts with its graphic realism and the seamless link from sci-fi to fantasy. Its obscene, but also entertaining and when you're done reading, you'll feel as dirty as Hari's audience should.Slaughterhouse Five, by Kurt Vonnegut Jr
Vonnegut. Weird. Just read it.Middle Passage, by Charles Johnson
Imagine a scifi-fantasy escapist novel about a black man who runs away and accidently joins the crew of a slave ship. That hardly begins to describe this novel which won a national book award, though I don't recall which one.Grendel, by John Gardner
This novel is written from the point of view of poor poor Grendel. See things from his point of view.

I like fairly long books, big battles, intricate plot, and I don't want to the book to be childish. I don't mind profanity and/or adult situations.
These should meet those conditions admirably:
Lord of the Rings (Tolkien)[The first 100 pages or so were written as for a children's book, the next 1000 are on an adult level]
A Game of Thrones (George R.R. Martin)[This is the first of what will be six books. The first three are out, and each is better than the last. This is a must read]
Dune (Frank Herbert)
Heroes Die + Blade of Tyshalle (Matthew W. Stover)[Well stated, DL]
The Eye of the World (Robert Jordan) [Not as good as these others, but it's a current favorite and it will be interesting to see how it compares.]

If you want some sci-fi books set in an extreemly realistic universe that read like Clancy novels, try the BattleTech line. Some good ones to start you out would be Stackpole's Warrior trillogy, and Wolves on the Border and Heir to the Dragon by Rob Charrette.

A Song of Ice and Fire by George R. R. Martin. I can?t possibly say enough good things about this series, but I?ll try to restrain myself. Tyrion Lannister alone makes this series worth reading, and in my opinion is quite possibly the greatest fictional character ever. Jon Snow and Daenerys Stormborn are great characters in their own rights, Jon especially. Martin has the most intricate plotline I?ve ever seen in a fantasy series. No ones agenda is quite what it seems, everyone who has an agenda is trying to further it, and everyone has an agenda. The series is quite bloody and grim, definitely not for the faint of heart. Nine year old girls calmly stab people, daughters watch their fathers get beheaded, sons kill their fathers, and nice guys get a blade through the heart for their troubles.
(currently books 1-3 are available out of a projected seven, so this might not help?)

The Lord of the Rings by J.R.R. Tolkien. It?s the Lord of the Rings. ?Nuff said. (the entire trilogy was originally intended to be one book, so really, it?s one book?). I?ll also very highly recommend the Silmarillion.

The Fionaver Tapestry OR Tigana by Guy Gavriel Kay. He?s somewhere between Tolkien and Martin in style. Fionaver is more fun, and Pwyll Twiceborn is one of my favorite fantasy characters, but by the time he wrote Tigana his writing style had noticeably improved. I?d pick one for your purposes (I?d chose Tigana since you only want one book), and then come back later to read the other.

The Wheel of Time by Robert Jordan. Fluff, but greatly entertaining fluff. The disproportionate amount of space on the Internet that Wheel of Time pages take up is a good indication of that. Jordan is a great world-builder, surpassing even Tolkien and Martin for the depth of his creation, and as a storyteller he tends to be rather expressive. Reading the Wheel of Time is the closest I?ve felt to watching a blockbuster film while reading. Scenes like the battle of Dumai?s Wells, and all of Rand?s battles, get the adrenaline flowing.
The first book, The Eye of the World, pretty much stands alone.

Fred Saberhagen's book of Swords series. great fantasy with big battles.
Timothy Zahn's Conquerors' Series, Sci-fi simmilar to NJO
L.Ron Hubbard's Battlefield Earth, Great Book long but great. The Movie was horrible and has little to nothing to do with the Book.

Seafort Saga by David Feintuch, rather different from the normal "future with starships".

Edited to say :
Jules Verne - went to the moon and the depths of the sea.
HG Wells - Time Travel and alien invasion.
To a certain extent Arthur Conan Doyles Professor Challenger books (Dinosaurs, a disintergrator machine and the balance of power, a very literal living Earth several decades before the Gaia hypothesis)

Conqueror's Heritage by Timothy Zahn.
The best SciFi book I've read to date. It is the second book of the series, and the book starts out in the perspective of the alien conqueror's, and stays there the entire book.

Dune by Frank Herbert. What SciFi list would be complete without Dune. Tough to get into but once you do, you're hooked.

The Stand by Stephen King. Personally, I consider this more scifi/fantasy than horror. A story about a virus that wipes out 99.9% of the population of Earth, with a battle between the forces of good and evil for the souls of those remaining. Considered by many to be the best Stephen King book.